Yes, most any savable parameter in a saved game can be edited to help produce a different result or outcome when you next load the game and press "continue current game". You can also bring a Player back from the dead (zero out wounds, increasing HP, or supply a spell that does the same).
However in real time only items that affect stats, bonuses or protection will make the game "easier". By this I mean an individual Party member will have a better chance of surviving via increased combat skill or other specific stats. Increasing Luck can significantly increase you combat prowess. Real time changing the game play difficulty (Easiest, Medium, Hardest) is not possible to my knowledge. It can be done in a saved game, but this will not affect most parameters as they are established at game creation (as the dungeon is being constructed).
Finding Quest items, unfortunately, has no apparent benefits other than "objective achieved". This is a short sight likely due to memory constraints. On the other hand you can create an off-play scoring system, e.g. in Halls or Sub12 that includes points for finding Quest items.